When multistage optical amplifiers are connected to conduct long-distance optical communication, there occurs amplified spontaneous emission (hereinafter referred to as `ASE`) from the multistage optical amplifiers themselves. This ASE is superposed on an optical signal while the optical signal is propagated from an optical transmitter to an optical receiver, thereby deteriorating the signal-to-noise ratio of the optical signal.
Conventionally, there are several methods of removing ASE: 1) a method of transmitting only the wavelength of signal light by a narrow-band pass filter (Japanese patent application laid-open No. 4-241328 (1992)); 2) a method of blocking ASE generated inside an optical amplifier by an optical switch during the OFF term of a signal light pulse (Japanese patent application laid open No. 5-5912 (1993)); 3) a method of inserting a polarization separating element into the output of an optical amplifier to extract signal light by using the linear polarization of signal light (Japanese patent application laid-open Nos. 6-196786 (1994) and 5-235442 (1993)); 4) a method of adding an ASE-absorbing element in the cladding of a rare earth element doped optical fiber as an amplifier medium to remove ASE propagating through the core (Japanese patent application laid-open No. 6-342175 (1994)); 5) a method of inserting an Er-doped optical fiber in the midst of an Er-A1-doped optical fiber to remove ASE by using an absorption characteristic of the Er-doped optical fiber (Japanese patent application laid-open No. 5-29683 (1993)); and 6) a method of removing ASE wavelength light by inserting a wave filter in the midst of a rare earth element doped optical fiber as an amplifier medium or by using bending losses of optical fiber to wavelength light and ASE wavelength light (Japanese patent application laid-open No. 4-86728 (1992)).
However, all these conventional methods relate to an optical amplifier for single-wavelength optical transmission and are not suitable for amplifying a wavelength multiplexed optical signal. Namely, in the method 1), the narrow-band pass filter has the characteristic of passing only light with a specific wavelength. Therefore, when several signal lights with different wavelengths are amplified, only one of the signal lights can be passed through, but the other signal lights cannot be passed through.
Also, in the method 2), when several signal lights with different wavelengths are used, an ON signal of one signal light may be canceled by switching off the optical switch in the OFF term of specific signal light since ON/OFF terms of signal pulse in the signal lights are different from each other.
Furthermore, in the method 3), the polarization states of lights to be propagated through an optical fiber cannot be kept stable unless an optical fiber with a particular structure is used. In general, in an optical fiber to be used as a transmission line, there may occur a stress due to the bending of the optical fiber and a variation of refractive index inside the optical fiber due to a change in surrounding temperature. Thus, there occurs a temporal fluctuation in the polarization direction of signal light propagating through the optical fiber. Moreover, the degree of fluctuation is different depending on wavelength. Therefore, in case of multi-wavelength optical transmission, it is substantially impossible that the polarization states of all wavelengths signal lights are unified.
In the methods 4), 5) and 6), ASE is absorbed or attenuated over the entire wavelength band of several tens nm including the signal light wavelength. Therefore, when the absorption or attenuation ratio of ASE is increased, the signal light wavelength must be also attenuated.
Thus, none of the conventional methods of removing ASE cannot give an improved signal-to-noise ratio in optical amplification of wavelength-multiplexed optical signal.